Is the chemical gate of connexins voltage sensitive? Behavior of Cx32 wild-type and mutant channels

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Abstract
Connexin channels are gated by transjunctional voltage (Vj) or CO2 via distinct mechanisms. The cytoplasmic loop (CL) and arginines of a COOH-terminal domain (CT1) of connexin32 (Cx32) were shown to determine CO2sensitivity, and a gating mechanism involving CL-CT1 association-dissociation was proposed. This study reports that Cx32 mutants, tandem, 5R/E, and 5R/N, designed to weaken CL-CT1interactions, display atypicalVjand CO2 sensitivities when tested heterotypically with Cx32 wild-type channels inXenopus oocytes. In tandems, two Cx32 monomers are linked NH2-to-COOH terminus. In 5R/E and 5R/N mutants, glutamates or asparagines replace CT1 arginines. On the basis of the intriguing sensitivity of the mutant-32 channel toVjpolarity, the existence of a “slow gate” distinct from the conventionalVjgate is proposed. To a lesser extent the slow gate manifests itself also in homotypic Cx32 channels. Mutant-32 channels are more CO2 sensitive than homotypic Cx32 channels, and CO2-induced chemical gating is reversed with relative depolarization of the mutant oocyte, suggestingVjsensitivity of chemical gating. A hypothetical pore-plugging model involving an acidic cytosolic protein (possibly calmodulin) is discussed.